Polystyrene is a well-known thermoplastic material finding a wide variety of uses. However, polystyrene by itself has a very low impact strength which limits its applications. There are two distinct and well-known processes to improve the impact strength of polystyrene. One process is a graft polymerization process wherein styrene monomer is grafted onto a rubbery polymer. Such a graft polymerization process is taught in U.S. Pat. No. 3,485,894 and British Pat. No. 1,230,507. An entirely different process for making high impact polystyrene involves the physical blending of a styrene polymer and a rubbery polymer.
There are a large number of patents claiming polystyrene blends having improved impact strength. U.S. Pat. No. 3,429,951 shows a blend of 75 parts by weight of polystyrene with 25 parts by weight of an AB block copolymer containing 25 weight percent of polymerized styrene and 75 weight percent of polymerized butadiene, the combination containing about 81 weight percent of total styrene. This reference teaches the necessity of a peroxide cure to achieve the desired properties; but there is a simple disclosure of such a combination which was momentarily free of peroxide in the course of preparation of a combination within the invention of the reference. However, the reference does not specify any critically for block molecular weight lengths nor for microstructure. U.S. Pat. No. 3,810,957, discloses blends of a hydrogenated styrene-diene block copolymer having at least two styrene end blocks and at least one hydrogenated diene mid block. However, the Izod impact strengths obtained by the blends of the '957 patent were relatively low except at very high rubber loading. Another patent disclosing polystyrene blends is U.S. Pat. No. 3,907,931 which claims blends of a graded styrene-butadiene AB block copolymer and polystyrene. The block copolymers used in the example of the '931 reference, however, have very high molecular weights--245,000 and 280,000 and 300,000--making them very difficult to produce and to blend. As with the '951 patent, the '931 patent does not mention any critically for microstructure of the block copolymer component. Still another polystyrene blend patent is U.S. Pat. No. 4,145,376, which claims blends of polystyrene, a rubbery diene polymer and a sequenced copolymer of styrene and a diene. Very high rubber loadings appear to be required in the '376 patent to obtain good impact. It is clear that the polymer compositions cited in the above references have a number of deficiencies which limit their commercial value.